In the aeronautical sector, electricity is gradually becoming predominant when it comes to energy over hydraulic or pneumatic energy. On-board aeronautical electrical networks are currently moving towards the use of DC current in combination with a high voltage level, as provided by new HVDC (High Voltage Direct Current) networks. Three-phase AC current (for example 115 VAC with a constant frequency), generated using turbines, is thus converted into a high DC voltage (for example +270 VDC/−270 VDC). To perform such a conversion operation, it is common to use an assembly formed of an autotransformer and of a rectifier bridge assembly, which assembly is able to be referred to under the name ATRU (‘Auto Transformer Rectifier Unit’). It will be recalled that an autotransformer is a particular type of transformer in which the whole winding performs the role of primary winding and the part of the winding up to the intermediate point performs the role of secondary winding; the primary winding and the secondary winding thus have a common part without any galvanic isolation between them. For equal nominal power, it is thus less bulky and less heavy than a conventional transformer, this being advantageous in aeronautical applications.
Positioning a rectifier based on capacitors at the output of the autotransformer would reinject, into the AC circuit, currents having frequencies that are harmonics of the frequency of the AC supply current. Thus, the value of the angular phase offset between the voltage and the intensity of the current (also called ‘cos phi’) would be impaired, as would the total harmonic distortion voltage (also called THDv). In order not to have to use a filtering cell with an autotransformer, and still reduce residual ripple of the DC current and the harmonics reinjected into the network, various solutions involving artificially creating one or two further three-phase networks, most often offset by 20°, 37° or 40°, in an autotransformer by using additional outputs have already been proposed. This structure may then be coupled to a twelve-pulse (for one three-phase network in addition to the main network) or eighteen-pulse (for two three-phase networks in addition to the main network) rectifier.
An autotransformer taking a three-phase voltage at input may, as is known, be represented by a vector diagram. The three input voltages of the three-phase AC network form an equilateral triangle whose centre is the neutral voltage point. The various output voltages may be represented by a vector whose origin is the centre of the triangle, the length of the vector representing the maximum amplitude of the output voltage, and the angle of the vector with respect to a reference vector representing the phase of the output voltage. The windings present on one and the same limb of the autotransformer, which are therefore coupled magnetically as they are flowed through by the same magnetic flux, are represented in parallel on the vector diagram by various segments. The electrical interconnections between the windings are represented on the vector diagram by segment intersections. The length of these segments represents the number of turns of the windings.
Document US 2002/0186112 A1 thus discloses an autotransformer that artificially recreates two three-phase networks in addition to the main network, the second three-phase network thus recreated being offset by a phase of between 35° and 40° (preferably 37°) with respect to the main network, the third recreated three-phase network being offset by a phase of between 35° and 40° (preferably 37°) with respect to the second recreated three-phase network. The amplitudes of the output voltages of the second and third networks are between 0.73 and 0.78 times the output voltage of the first main network, preferably equal to 0.767 times the output voltage of the first main network. The autotransformer is moreover coupled to an 18-pulse rectifier. The document therefore describes what is termed a ‘37° ’ topology. However, the autotransformer described in this document does not provide a solution for containing the mass of the autotransformer. Now, this parameter is decisive in aeronautical applications. Manufacturers commonly change the nominal power rating of the autotransformer, at the expense of a slight impairment of the efficiency, which is compensated for by a high-performance cooling system that most often uses cold plates or specific heat sinks that exhibit a compromise between extraction power and high mass. The problem with this rating choice is the use of cooling solutions that are complex and expensive and even, in the case of the cold plates, difficult to integrate into an already existing system given the complexity of the modifications to be made.